The hematopoietic microenvironment is of major importance in vivo in supporting lodgement, proliferation, and differentiation of hematopoietic progenitor cells in a given organ. Hematopoiesis also can be maintained for several months in vitro in the presence of a supportive adherent stroma. Endothelial cells, adventitial reticular cells, macrophages, fibroblasts, and fat cells are the main cellular components of both normal and pathologic hematopoietic stroma. Occasional minor cellular differences have been noted between areas supporting erythropoiesis vs. granulocytopoiesis. Extracellular stroma plays a major tissue specific role in supporting epithelial cell differentiation. The purpose of these studies is to examine the composition of the extracellular stromal matrix of hematopoietic tissue in normal and pathologic conditions and to determine its role in supporting hematopoietic proliferation and differentiation. We will study normal mice, S1/S1d mutants, which have a defective hematopoietic microenvironment, and several natural and induced hematologically pathologic states. We will use marrow, spleen, and non-hematopoietic organs from normal and abnormal rodents to analyze the major components of the stroma, including collagen sub-types, fibronectin, laminin, glycosaminoglycans and proteoglycans. In addition, the stroma of long term hematopoietic cultures of cells from these animals will be analyzed to determine the synthesis and location of the various stromal components. These findings will be correlated with the level of hematopoiesis and lines of differentiation supported in the cultures. Finally, extracellular stromal matrix material will be extracted from marrow, spleen, and non-hematopoietic organs. These biomatrices will be placed in culture dishes and tested for their effects on maintenance of hematopoiesis in long term cultures. These studies should give the first in depth insight into biochemial composition of hematopoietic microenvironmental tissue and its relevance to maintenance of hematopoiesis and regulation of hematopoietic differentiation. These studies have broad relevance to hematology, developmental biology, and to cell biology in general.